Heart failure (HF) is a major and growing public health problem. In the United States for example approximately 5 million patients have HF and over 550 000 patients are diagnosed with HF for the first time each year (In: American Heart Association, Heart Disease and Stroke Statistics: 2005 Update, Dallas, Tex.; American Heart Association (2005)). Similarly US-statistics show that HF is the primary reason for 12 to 15 million office visits and 6.5 million hospital days each year. From 1990 to 1999, the annual number of hospitalizations has increased from approximately 810 000 to over 1 million for HF as a primary diagnosis and from 2.4 to 3.6 million for HF as a primary or secondary diagnosis. In 2001, nearly 53 000 patients died of HF as a primary cause. Heart failure is primarily a condition of the elderly, and thus the widely recognized “aging of the population” also contributes to the increasing incidence of HF. The incidence of HF approaches 10 per 1000 in the population after age 65. In the US alone, the total estimated direct and indirect costs for HF in 2005 were approximately $27.9 billion and approximately $2.9 billion annually is spent on drugs for the treatment of HF (cf. the above cited AHA-statistics).
Heart Failure
Heart Failure is characterized by a loss in the heart's ability to pump as much blood as the body needs. Failure does not mean that the heart has stopped pumping but that it is failing to pump blood as effectively as it should.
The NYHA (New York Heart Association) and the ACC/AHA (American Association of Cardiology/American Heart Association) have both established functional classes of HF to gauge the progression of the disease. The NYHA classification scheme has four classes of disease state: Class 1 is asymptomatic at any level of exertion. Class 2 is symptomatic at heavy exertion and Classes III and IV are symptomatic at light and no exertion, respectively.
In the four stage ACC/AHA scheme, Stage A is asymptomatic but is at risk for developing HF. Stage B there is evidence of cardiac dysfunction without symptoms. In Stage C there is evidence of cardiac dysfunction with symptoms. In Stage D, the subject has symptoms of HF despite maximal therapy.
Etiology of HF
Medically, heart failure (HF) must be appreciated as being a complex disease. It may be caused by the occurrence of a triggering event such as a myocardial infarction (heart attack) or be secondary to other causes such as hypertension, diabetes or cardiac malformations such as valvular disease. Myocardial infarction or other causes of HF result in an initial decline in the pumping capacity of the heart, for example by damaging the heart muscle. This decline in pumping capacity may not be immediately noticeable, due to the activation of one or more compensatory mechanisms. However, the progression of HF has been found to be independent of the patient's hemodynamic status. Therefore, the damaging changes caused by the disease are present and ongoing even while the patient remains asymptomatic. In fact, the compensatory mechanisms which maintain normal cardiovascular function during the early phases of HF may actually contribute to progression of the disease in the long run, for example by exerting deleterious effects on the heart and its capacity to maintain a sufficient level of blood flow in the circulation.
Some of the more important pathophysiological changes which occur in HF are (i) activation of the hypothalamic-pituitary-adrenal axis, (ii) systemic endothelial dysfunction and (iii) myocardial remodeling.
(i) Therapies specifically directed at counteracting the activation of the hypothalamic-pituitary-adrenal axis include beta-adrenergic blocking agents (B-blockers), angiotensin converting enzyme (ACE) inhibitors, certain calcium channel blockers, nitrates and endothelin-1 blocking agents. Calcium channel blockers and nitrates, while producing clinical improvement have not been clearly shown to prolong survival, whereas B-blockers and ACE inhibitors have been shown to significantly prolong life, as have aldosterone antagonists. Experimental studies using endothelin-1 blocking agents have shown a beneficial effect.
(ii) Systemic endothelial dysfunction is a well-recognized feature of HF and is clearly present by the time signs of left ventricular dysfunction are present. Endothelial dysfunction is important with respect to the intimate relationship of the myocardial microcirculation with cardiac myocytes. The evidence suggests that microvascular dysfunction contributes significantly to myocyte dysfunction and the morphological changes which lead to progressive myocardial failure.
In terms of underlying pathophysiology, evidence suggests that endothelial dysfunction may be caused by a relative lack of NO which can be attributed to an increase in vascular O2-formation by an NADH-dependent oxidase and subsequent excess scavenging of NO. Potential contributing factors to increased O2-production include increased sympathetic tone, norepinephrine, angiotensin endothelin-1 and TNF-α. In addition, levels of IL-10, a key anti-inflammatory cytokine, are inappropriately low in relation to TNF-α levels. It is now believed that elevated levels of TNF-α, with associated proinflammatory cytokines including IL-6, and soluble TNF-α receptors, play a significant role in the evolution of HF by causing decreased myocardial contractility, biventricular dilatation, and hypotension and are probably involved in endothelial activation and dysfunction. It is also believed that TNF-α may play a role in the hitherto unexplained muscular wasting which occurs in severe HF patients. Preliminary studies in small numbers of patients with soluble TNF-receptor therapy have indicated improvements in NYHA functional classification and in patient well-being, as measured by quality of life indices.
(iii) Myocardial remodeling is a complex process which accompanies the transition from asymptomatic to symptomatic heart failure, and may be described as a series of adaptive changes within the myocardium, like alterations in ventricular shape, mass and volume (Piano, M. R., et al., J. Cardiovasc. Nurs. 14 (2000) 1-23, quiz 119-120; Molkentin, J. D., Ann. Rev. Physiol. 63 (2001) 391-426). The main components of myocardial remodeling are alterations in myocyte biology, like myocyte hypertrophy, loss of myocytes by necrosis or apoptosis, alterations in the extracellular matrix and alterations in left ventricular chamber geometry. It is unclear whether myocardial remodeling is simply the end-organ response that occurs following years of exposure to the toxic effects of long-term neurohormonal stimulation, or whether myocardial remodeling contributes independently to the progression of heart failure. Evidence to date suggests that appropriate therapy can slow or halt progression of myocardial remodeling.
Markers and Disease State
As indicated above, myocyte hypertrophy is likely to represent one of the first steps down the road to HF. Myocyte hypertrophy is characterized by an increased expression of some genes encoding contractile proteins, such as p-myosin heavy chain and troponin T (TnT), and of some non-contractile proteins, such as A-type and B-type natriuretic peptides, by an increased cell size and by cytoskeletal alteration (Piano, M. R., et al., J. Cardiovasc. Nurs. 14 (2000) 1-23, quiz 119-120; Molkentin, J. D., Ann. Rev. Physiol. 63 (2001) 391-426).
Studies of human and animal models of heart failure suggest depressed myocyte function in the later stages of cardiac failure. The mechanisms that underlie myocyte dysfunction have been suggested to involve alterations in the calcium-handling network, myofilament and cytoskeleton (de Tombe, P. P., Cardiovasc. Res. 37 (1998) 367-380). For example, in human and animal models of heart failure, sarcoplasmic reticulum calcium-ATPase enzyme activity is reduced, while both mRNA and protein levels of the sarcolemmal Na+/Ca2+ exchanger are increased. Moreover, there is isoform-switching of TnT, reduced phosphorylation of troponin I (TnI), decreased myofibrillar actomyosin ATPase activity and enhanced microtubule formation in both human, and animal models of heart failure.
Initially the changes to the heart, leading to myocardial remodeling are meant to compensate for the diseased parts of the myocardium in order to sustain the body's demand for oxygen and nutrients. However, the compensatory phase of heart failure is limited, and, ultimately, the failing heart is unable to maintain cardiac output adequate to meet the body's needs. Thus, there is a transition from a compensatory phase to a decompensatory phase. In the decompensatory phase, the cascade of changes in the heart continues but is no longer beneficial, moving the patient down the progression of heart failure to a chronic state and eventual death.
According to the “ACC/AHA 2005 Guideline Update for the Diagnosis and Management of Chronic Heart Failure in the Adult”, the disease continuum in the area of heart failure is nowadays grouped into four stages as noted above. In stages A and B the individuals at risk of developing heart failure are found, whereas stages C and D represent the groups of patients showing signs and symptoms of heart failure. Details for defining the different stages A through D as given in the above reference are hereby included by reference.
Diagnostic Methods in Heart Failure
The single most useful diagnostic test in the evaluation of patients with HF is the comprehensive 2-dimensional echocardiogram coupled with Doppler flow studies to determine whether abnormalities of myocardium, heart valves, or pericardium are present and which chambers are involved. Three fundamental questions must be addressed: 1) is the LVEF preserved or reduced, 2) is the structure of the LV normal or abnormal, and 3) are there other structural abnormalities such as valvular, pericardial, or right ventricular abnormalities that could account for the clinical presentation? This information should be quantified with a numerical estimate of EF, measurement of ventricular dimensions and/or volumes, measurement of wall thickness, and evaluation of chamber geometry and regional wall motion. Right ventricular size and systolic performance should be assessed. Atrial size should also be determined semiquantitatively and left atrial dimensions and/or volumes measured.
Noninvasive hemodynamic data acquired at the time of echocardiography are an important additional correlate for patients with preserved or reduced EF. Combined quantification of the mitral valve inflow pattern, pulmonary venous inflow pattern, and mitral annular velocity provides data about characteristics of LV filling and left atrial pressure. Evaluation of the tricuspid valve regurgitant gradient coupled with measurement of inferior vena caval dimension and its response during respiration provides an estimate of systolic pulmonary artery pressure and central venous pressure.
Stroke volume may be determined with combined dimension measurement and pulsed Doppler in the LV outflow tract. However, abnormalities can be present in any of these parameters in the absence of HF. No one of these necessarily correlates specifically with HF; however, a totally normal filling pattern argues against clinical HF.
From a clinical perspective, the disease is clinically asymptomatic in the compensatory and early decompensatory phases (completely asymptomatic in stage A and with structural heart disease but no signs and symptoms of HF in stage B, cf. the ACC/AHA practice guidelines). Outward signs of the disease (such as shortness of breath) do not appear until well into the decompensatory phase (i.e., stages C and D according to the ACC/AHA guidelines). Current diagnosis is based on the outward symptoms of patients in stages C and D.
Typically patients with heart failure receive a standard treatment with drugs that interact with specific mechanisms involved in heart failure. There are no diagnostic tests that reflect those specific mechanisms reliably and help the physician to choose the right drug (and dose) for the right patient (e.g., ACE inhibitor, AT II, β-blockers, etc).
Prior Diagnosis of HF with Markers
Early assessment of patients at risk for heart failure appears to be possible only by biochemical markers since the individual at risk of developing heart failure at that stage is still free of clinical HF symptoms. There are no established biochemical markers currently available for the reliable pre-symptomatic assessment of the disease. By the time the diagnosis HF is established nowadays, the disease is already well underway.
The natriuretic peptide family, especially the atrial natriuretic peptide family and the brain natriuretic peptide family have in recent years proven to be of significant value in the assessment of HF.
HF Prognosis and Need
At least partially due to the late diagnosis, 50% of patients with HF die within two years of diagnosis. The 5-year survival rate is less than 30%. There is a significant need for new biochemical markers aiding in the early diagnosis of heart failure.
An improvement in the early assessment of individuals at risk for heart failure, i.e., of individuals that are clinically asymptomatic for heart failure is warranted.
It has been established in recent years that B-type natriuretic peptide markers represent an excellent tool to monitor disease progression in patients with HF and to assess their risk of cardiovascular complications, like heart attack.
However, as for many other diagnostic areas a single marker is not sufficient.
Whereas a low value of NT-proBNP has a very high negative predictive value for ruling out HF or LVD, the positive predictive value for heart failure in the above and other studies (cf. Triepels, R. H., et al., Clin. Chem. 49, Suppl. (2003) A37-38) has been found to be in the range of 50-60%. Thus a marker useful in assessing individuals at risk for heart failure that on its own e.g., has a high, or in combination with NT-proBNP, and as compared to NT-proBNP alone has an improved positive predictive value for HF is of high clinical/practical importance.
A marker aiding in the assessment of a patient with heart failure also is of high importance to achieve further technical progress in this clinically very important and demanding diagnostic area.